The present invention relates to an image reading apparatus provided with a color image pick-up system.
An image reading apparatus reads an image of a document, typically by moving a carriage having a color image pick-up system in parallel with the document surface. The pick-up system is equipped with a line sensor, made, for instance, of three kinds of CCDs for reading three primary colors, each being constituted by a large number of linearly arrayed image pick-up elements.
In the case of a flat bed type image reading apparatus, a document bed formed of a transparent plate such as glass is provided on the upper surface of a box type casing to place the document thereon. A carriage inside the casing is moved in parallel with the document bed by the action of a driving device. A light source and a color image pick-up system are installed on this carriage. The irradiation light of the light source is reflected by the document surface on the bed, and converged onto the color image pick-up system by a condenser lens.
In order to increase resolution of an image read in the main scanning direction, i.e. a direction in which the image pick-up elements are arrayed in a CCD, the image reading apparatus is required to have the increased number of image pick-up elements constituting the CCD. This, however, increases the size of the CCD as well as the cost associated with optical design therefor, if the number of the pick-up elements is simply increased without changing the size of the individual element. Further, making the pick-up element smaller in size has a limitation of manufacture.
As an attempt to meet the above requirement, JP-A-58-19081 discloses a CCD image sensor in which optical detectors are arranged in first and second rows with the second row optical detector being shifted by about half width of the individual optical detecting element relative to the first row optical detector. In this CCD image sensor, the first low optical detector and the second row optical detector are arranged adjacent to each other in the sub-scanning direction.
The CCD image sensor having such plural rows of optical detectors as disclosed makes it possible to realize twice reading resolution in the main scanning direction in comparison to a CCD image sensor having a single row of an optical detector as the same line on a document can be read by both of the first row optical detector and the second row optical detector.
A problem associated with the CCD image sensor having the plural rows of optical detectors reading the same line on the document is a level difference that may occur between adjacent pixel outputs (the output of the odd numbered pixel and the output of the even numbered pixel) on a line due to the difference of characteristics between the first and second row optical detectors, and the difference of characteristics between respective shift registers to which electric charges are transferred from optical detectors. This level difference, typically when the image signal is processed at the high resolution not lower than a 128-level gray scale, results in longitudinal stripes on a read-out image, greatly degrading image quality.
Since the longitudinal stripes conspicuously appear on pixel data in a dark portion rather than on pixel data in a bright portion, the longitudinal strips give a serious problem particularly to the pixel data in the dark portion. Herein, the pixel data in the dark portion mean, for example, in the case of 256-level gray scale as shown in FIG. 6, the pixel data of the level not higher than about an output level 20 in a reflection-type document and a transmission-type document positive film, or the pixel data of the level not lower than about an output level 230 in a transmission-type document negative film. That is, the pixel data in the dark portion have a value indicative of a level darker than a predetermined level. Further, the pixel data in the bright portion mean the pixel data of the level not lower than about the output level 20 in the reflection-type document and the transmission-type document positive film, or the pixel data of the level not higher than about the output level 230 in the transmission-type document negative film. That is, the pixel data in the bright portion have a value indicative of a level brighter than a predetermined level.
A primary object of the present invention is to provide an image reading apparatus which can effectively correct a level difference between adjacent pixel outputs (odd numbered pixel output and even numbered pixel output) on each one line.
Another object of the present invention is to provide an image reading apparatus which can increase the resolution in the main scanning direction, and which can read an image with the low resolution in the sub-scanning direction at high speed.
In an image reading apparatus according to a first aspect of the present invention, a color image pick-up system has image pick-up element groups. In each of the groups, a plurality of image pick-up element rows are arranged in parallel one another on a substrate. In each of the image pick-up element rows, a plurality of image pick-up elements are linearly arranged. The image pick-up element rows in the same group are shifted one from another by an amount smaller than the width of one image pick-up element in the direction in which the image pick-up elements are linearly arrayed. A first comparison circuit generates a signal to select the this-time pixel data when the differential value of the this-time pixel data and the last time pixel data for each one line is larger than a predetermined setting value. When the differential value is not larger than a predetermined setting value, the first comparison circuit generates a signal to select the output data of the averaging circuit. The first comparation circuit sends either of the signals to the selection circuit. Further, the second comparison circuit generates a signal to select the this-time pixel data, when the this-time pixel data have a value indicative of a level brighter as an image than a predetermined level. The second comparison circuit generates a signal to select the output signal of the first comparison circuit when the this-time pixel data have a value indicative of a level darker than a predetermined level. The second comparison circuit sends either of the signals to these election circuit. Accordingly, for the image data in the bright portion, the this-time pixel data is selected. For the pixel data in the dark portion, the output signal of the first comparison circuit is selected. Therefore, only for the pixel data in the dark portion, the level difference between the odd numbered pixel output and the even numbered pixel output for each one line is dissolved. Consequently, the level difference between the odd numbered pixel output and the even numbered pixel output for each one line can be effectively dissolved.
In an image reading apparatus according to a second aspect of the present invention, the first comparison circuit has a first set section to set a predetermined setting value, a differential unit to calculate the differential value of the this-time pixel data and the last time pixel data for each one line, and a first comparator to compare the differential value calculated by the differential unit with the setting value set by the set section. Further, the second comparison circuit has a second set section to set a predetermined level value, and a second comparator to compare the this-time pixel data with the level value set by the second set section. Accordingly, the level difference between the odd numbered pixel output and the even numbered pixel output for each one line can be appropriately and surely dissolved. Consequently, a problem that edges, or the like, of the image are blurred, can be prevented.
In an image reading apparatus according to a third aspect of the present invention, the shading correction circuit conducts the shading correction of the pixel output data of the A/D converter. Accordingly, the level difference caused between the odd numbered pixel output and the even numbered pixel output after the shading correction has been conducted, can be dissolved.
In an image reading apparatus according to a fourth aspect of the present invention, the color image pick-up system has the image pick-up element rows which are arranged at the pitch of integer times, not smaller than two times, of the height of the image pick-up element in the direction vertical to the arrayed direction of the image pick-up elements. Accordingly, when the color image pick-up system is relatively moved to the document in the sub-scanning direction, which is perpendicular to the main-scanning direction, at integer times of the speed, all of the image pick-up element rows read the same line on the document. Consequently, the high speed reading in the low resolution can be achieved.
In an image reading apparatus according to a fifth aspect of the present invention, since the image pick-up element groups correspond respectively to red, green and blue, the light from the document can be read while being separated into three primary colors.
In an image reading apparatus according to a sixth aspect of the present invention, because, in each the image pick-up element group, the first and the second element rows are shifted from each other by almost a half pitch of the width of the image pick-up element, the resolution of reading in the main-scanning direction can be twice.
In an image reading apparatus according to a seventh aspect of the present invention, because the image pick-up element group has an opening portion not larger than the light receiving area of the image pick-up element and a shield portion to shield the light from reaching onto the peripheral portion of the image pick-up element, the range which is read in an overlapped manner by a plurality of image pick-up elements on the document, can be reduced. Consequently, the resolution of reading can be increased substantively.
The present disclosure relates to the subject matter contained in Japanese patent application Nos. Hei. 11-36223 (filed on Feb. 15, 1999) and Hei. 11-358736 (filed on Dec. 17, 1999), which are expressly incorporated herein by reference in their entireties.